genetic diversity + natural selection

• Genetic diversity is a measure of the number of different alleles of genes within a population
• genetic diversity is increased by:

  • dna mutation (give rise to new alleles)
  • Migration of individuals with different alleles from another population which reproduce = gene flow.
• all members of the same species have the same genes but differ in their combination of alleles
• the greater number of different alleles that all number of a species posses the greater the genetic diversity
• genetic diversity allows natural selection to occur which in turn leads to the evolution in populations
• evolution is the change in allele frequency over many generations in a population
• process of natural selection:

  1. new alleles for a gene is created by random mutations
  2. if the new allele increases chance of survival in that environment they are more likely to survive and reproduce (natural selection)
  3. reproduction passes in advantageous alleles
  4. as a result over many generations the new allele increases in frequency in the population (evolution)
• natural selection results in a species becoming better adapted to their environment - could be: anatomical, physiological or behavioural
• polygene is when number of genes act together that are more influenced by the environment (producing individuals in a population that vary about the mean) rather than ones determined by a single gene/monogene
• Natural selection can occur in different ways – it might push a phenotype in a particular direction (e.g. necks of giraffes increasing in length) or it may make the mean phenotype (e.g. birth weight) more common.
• Directional selection

  • Alleles for an extreme phenotype are more likely to be selected for

  (one phenotype is selected over another)

  • Usually triggered by a change in the environment

  (e.g. antibiotic resistance)

  
• directional selection - antibiotic resistance with presence of antibiotic an environmental change

  • mutation in a bacterium enables it to develop resistance to an antibiotic.
  • When antibiotic is used it puts directional selection pressure on bacterial population, the resistant bacterium survives while the rest are killed.
  • The bacterium can grow without competition and reproduce, passing on its resistance allele to its offspring.
  • This produces a population of bacteria which carry the antibiotic resistance allele

  
• Stabilising selection

  • Alleles for an average phenotype are more likely to be selected for

  (any extremes of phenotype are selected against)

  • Occurs when there is no change in the environment
  • Reduces the range of possible phenotypes.

  (e.g. human birth weight)

  
• stabilising selection - alleles too low or high BW less likely to be passed on so BW stabilises towards the mean with less variation at the extremes

  • Underweight babies are less likely to survive as they lose a lot of body heat.
  • Overweight babies are more likely to lead to childbirth complications.
  • Alleles for a mean birth weight are more likely to survive and reproduce, passing on their average weight alleles onto their offspring.
• A genetic bottleneck is an event that causes a big reduction in a population, reduces the number of different alleles in the gene pool and so reduces genetic diversity. The survivors reproduce and a larger population is created from a few individuals.
• The founder effect is when a small group from the original population establishes a new population, resulting in a limited gene pool, reduced genetic variation, and a higher risk of genetic disease due to a non-random gene sample.
• Genetic drift is the change in the frequency of an existing gene variant in a population due to random chance